Have a look at the video I recently posted in a thread a couple down from this, re:Huntsman circa 1970.
http://uk.youtube.com/watch?v=so5Ge9CNEV4 I assume that is the sort of performance you want. I have helicopters, and experimented with lipo's in this boat. The problem is, making sure you do not overdischarge, if the individual cells drop below 3v, the lipo is finished. With a helicopter, its easy to tell when the volts are dropping, not as easy in a boat. You would need a very good low voltage alarm, with enough time to get back to shore.
I changed a stock brushed motor to brushless ourunner in a helicopter(the outer can turns, so you cant fit a water jacket). It was a big improvement, but can be expensive, you need a esc also, standard one will not work. They are far more efficient, no sparking brushes, but have not come across one yet specifically designed for boats, helis are light, and only fly for 10 mins on a 3 cell 11.1v lipo, so there is run time to consider also.
HS93 (forum member) has 2 brushless in the same boat as mine in the video, but I dont know if he has run it yet, maybe pm him for info